This invention relates to a process for making a thin film of solid material. In particular it relates to the production of a thin film made of a semi-conducting material, for example such as silicon.
Document FR-A-2 681 472 (corresponding to U.S. Pat. No. 5,374,564) discloses a process for manufacturing thin films made of a semi-conducting material. This document divulges that implantation of a rare gas and/or hydrogen in a substrate made of a semi-conducting material could create a layer that could contain micro-cavities or micro-bubbles (or platelets) at a depth approximately equal to the average penetration depth of the implanted ions. The implanted face of this substrate is brought into intimate contact with a support acting as a stiffener. Furthermore, heat treatment can be applied at a sufficiently high temperature to induce an interaction (or coalescence) between the micro-cavities or micro-bubbles causing a separation of the semi-conducting substrate into two parts, namely a thin semi-conducting film bonding to the stiffener, and secondly the rest of the semi-conducting substrate. The separation takes place at the location at which the micro-cavities or micro-bubbles are present, in other words along the micro-cavities layer. The heat treatment is such that the interaction between the micro-bubbles or the micro-cavities created by implantation causes a separation between the thin film and the rest of the substrate. Therefore, there is a transfer of a thin film from an initial substrate as far as a stiffener that acts as a support for this thin film.
This process may also be applied to the manufacture of a thin film made of solid material other than a semi-conducting material (a conducting or dielectric material) that may or may not be crystalline. This film may be single layer or multi-layer.
Thus, the implantation of gaseous compounds can create in-depth cavities or micro-bubbles or micro-cracks that will form a weakened layer close to the depth at which the ions stop. The implanted zone is more or less fragile depending on the nature and implantation conditions. They are chosen such that the implanted surface of the substrate is not deformed in any way. If any deformations in this surface occur in the form of blisters, these deformations will cause excessive weakening of the implanted zone.
Document FR-A-2 681 472 describes how, in order to transfer a thin film onto a support, the implanted substrate and the support (or stiffener) have to be bonded together before causing separation of the thin film from its original substrate, this separation possibly being caused by a heat treatment and/or a mechanical treatment (as described in document FR-A-2 748 851). Bonding is achieved by putting the implanted substrate and the support into intimate contact by means of molecular bonding, or a glue or an intermediate compound (insulating layer, conducting layer, etc.). This bonding is only possible if there are no deformations on the implanted surface, and therefore if no blisters have occurred.
In some cases, it is impossible to bond the implanted substrate and the support acting as a stiffener, particularly due to different coefficients of thermal expansion. It is also possible that the bonding forces are not sufficient to cause the stiffening effect. Therefore a thin film, for example a mono-crystalline film, can be obtained on any support using a process derived from the process divulged by document FR-A-2 681 472, for example the process divulged by document FR-A-2 738 671 (corresponding to U.S. Pat. No. 5,714,395). According to this process, the implanted gaseous compounds rust be at a sufficient depth and/or a layer of a material able to make the structure sufficiently rigid to obtain separation at the implanted zone must be deposited after the implantation step, in order to separate the thin film from its original substrate. The film obtained is then self supporting.
For the two processes mentioned above, the surface roughness of the thin film after transfer is variable depending on the implantation and/or separation conditions (heat and/or mechanical treatment) used to obtain this separation. In this case it may be useful to further weaken the zone containing the cavities. Separation would then be easier than in the normal case, in other words separation would be possible by applying lower mechanical forces and and/or a smaller thermal budget. This is particularly useful for structures composed of materials with different coefficients of thermal expansion and for which there are limiting heating temperatures.
The various means of weakening the implanted zone include an increase in the dose of implanted gaseous compounds and/or carrying out a heat treatment that may correspond to the heat treatment divulged in document FR-A-2 681 472. However, as mentioned above, the implanted dose and/or the thermal budget need to be limited before the bonding step in order to prevent deformations of the implanted surface.
Thus, there is no acceptable means of further weakening the implanted zone before applying the separation step. The existence of such a means would make it possible Go reduce thermal budgets and/or the mechanical forces necessary for separation. Thus, thin films could be transferred onto supports that cannot resist high temperatures, by using the process divulged in document FR-A-2 681 472. It would also be possible to more easily separate thick films using the process described in document FR-A-2 738 671. These thick films could then be transferred onto any type of support, even supports for which it would be impossible to obtain high bonding forces between the film and the support. Furthermore, increased weakening of the implanted zone would make it possible to reduce the roughness of the free surface of the film after transfer, while encouraging fracture.
Therefore, the problem that arises is to further weaken the implanted zone without inducing any blisters on the implanted surface of the original substrate.
The invention provides a solution to this problem. It is proposed to apply pressure on the implanted face of the substrate, at least during part of the coalescence of micro-cavities, in order to facilitate this coalescence and prevent implanted gaseous compounds from escaping from the substrate. The result is that weakening is increased.
Therefore, the purpose of the invention is a process for making a thin film from a substrate of a solid material with a plane face, comprising:
the implantation of gaseous compounds in the substrate to form a layer of micro-cavities located at a depth from the said plane face corresponding to the thickness of the required thin film, the gaseous compounds being implanted under conditions that can weaken the substrate at the layer of micro-cavities,
partial or complete separation of the thin film from the rest of the substrate, this separation comprising a step in which thermal energy is input and in which pressure is applied to the said plane face.
The xe2x80x9cMechanistic Studies of Silicon Wafer Bonding and Layer Exfoliationxe2x80x9d document by M. K. WELDON et al., published in Electrochemical Society Proceedings, volume 97-36, specifies that application of a compression stress on a glued structure composed of an implanted substrate and a stiffener is a means of closing micro-cracks and preventing exfoliation, while a uniform external tension can cause exfoliation at a lower temperature. It also mentions that application of a uniform pressure at lower temperatures is a means of developing more uniform micro-cracks such that a more uniform exfoliation can be obtained when the pressure is released and heat is applied. In this document, the applied pressure is a means of obtaining uniform micro-cracks but does not provide any information about increased weakening of the implanted zone by an increase in the size of micro-cracks. Thus, in this document, exfoliation is achieved by releasing the pressure and applying heat at a temperature a priori greater than the temperature used when the pressure was applied. In this document, unlike the case in this invention, the applied pressure is not used to increase weakening of the implanted zone and therefore to reduce the thermal budget and/or mechanical forces in order to obtain the thin film. Furthermore according to this invention, separation may be achieved under pressure. Moreover, according to one advantageous embodiment of the invention, the applied pressure may be adjusted during the process depending on changes to the gaseous phases present in the micro-cavities.
Gaseous compounds for the purposes of this description means elements, for example hydrogen or rare gases, either in their atomic form (for example H) or in their molecular form (for example H2) or in their ionic form (for example H+, H+2) or in their isotopic form (for example deuterium) or in isotopic and ionic form.
Furthermore, ionic implantation refers to any type of introduction of the previously defined compounds alone or in combination, such as ionic bombardment, diffusion, etc.
Regardless of the type of solid material, thermal energy causes coalescence of micro-cavities or micro-cracks which causes weakening of the structure at the micro-cavities layer. This weakening enables separation of the material under the effect of internal stresses and/or pressure in the micro-cavities, and this separation may be natural or may be assisted by the application of external stresses.
Applying pressure is a means of causing coalescence of micro-cavities while preventing the formation of blisters on the plane face. This pressure depends on the state of stress in the implanted zone.
Partial separation means separation in which attachment points are left between the thin film and the rest of the substrate.
The said pressure may be a gas pressure and/or a mechanical pressure applied for example by a piston. It may be applied locally or uniformly over the plane face.
The process may also comprise bonding of a thickener on the said plane face, after implantation of gaseous compounds. The thickener may consist of a wafer that may, for example, be bonded with the said plane face by molecular bonding. The thickener may also be composed of a deposit of one or several materials. Pressure can then be applied through the thickener. This thickener acts as a stiffener. In this case, the pressure that encourages coalescence of micro-cavities and prevents the formation of blisters takes account of the thickener. The thickener can induce stresses on the structure, encouraging the coalescence of micro-cavities.
Advantageously, while some of the micro-cavities are coalescing, the said pressure as adjusted to remain slightly higher than a pressure called the limiting pressure, below which blisters appear on the said plane face and above which blisters do not appear on the said plane face. This avoids applying unnecessarily high pressures.
The limiting pressure changes in time as the coalescence of micro-cavities varies. Thus, the pressure used according to the invention may be the maximum limiting pressure or it may be a limiting pressure that is applied gradually during the process and that varies as a function of the coalescence of the micro-cavities that in particular depends on the thermal budget (time, temperature). Therefore, the limiting pressure depends on the thermal budget. Thus, for example, for annealing a 300 nm Si film and a 5 xcexcm SiO2 film at 450xc2x0 C. for a given duration, a pressure of the order of a few bars has to be applied in order to achieve separation, whereas if there is no additional pressure (in other words at atmospheric pressure) annealing at more than 470xc2x0 C. is necessary to achieve separation and obtain a film.
Coalescence may be achieved such that the thin film can be separated from the rest of the substrate simply by pulling them apart.
According to another embodiment, the thin film is separated from the rest of the substrate by applying a heat treatment and/or mechanical forces.
The initial substrate may be a substrate that has already been used to make a thin film according to the process. For example, this substrate that has already been used may be polished to provide a new plane face.
The substrate may comprise one or several homogenous and/or heterogeneous layers on the side of the said plane face. It may be composed of a semi-conducting material, at least on the side of the said plane face. It may comprise all or part of at least one electronic device and/or at least one electro-optical device, on the side of the said plane face.
Due to the applied pressure, the invention can result in thinner self supporting films than are possible with a process without pressure. This pressure prevents the relaxation of micro-cavities in the form of blisters and enables these micro-cavities to interact to cause separation.
The invention can also be used to delay separation of the thin film by the use of an additional step that consists of applying additional pressure on the thin film.